Semiconductor diode lasers have been used extensively as transmitters for fiber-optic communications. In one common and low-cost implementation, edges of two-opposing end facets of a laser are cleaved to form resonant-reflective surfaces and provide the feedback necessary for laser operation. Such Fabry-Perot (FP) lasers typically emit in multiple longitudinal modes and have large output bandwidths, for example, 3 nanometers (nm) to 150 nm. Output bandwidths as large as about 400 nm are likely in the near future. In another common implementation with slightly increased complexity, a Bragg grating is formed in or adjacent the active region of the FP laser cavity to form a distributed feedback (DFB) laser. DFB lasers have the advantage of producing emission in a single longitudinal mode, which typically has a very narrow bandwidth, for example, less than 0.01 nm. In a third application, a distributed Bragg reflector (DBR) is substituted for each of the cleaved facets of the FP laser. The distributed Bragg reflector causes the laser to emit in a single longitudinal mode.
In advanced Dense-Wavelength-Division Multiplexing (DWDM) fiber-optic communication technology, optical signals each allocated to a different one of many closely-spaced channels are transmitted simultaneously on a single optical fiber. Typical spacings of the channels can range from about 5 nm to as little as 0.4 nm. Closer channel spacings are envisioned. To provide effective DWDM systems, stable and accurate transmitters of predetermined wavelengths are needed for the individual channels. In addition, stable and accurate wavelength-selective receivers are needed to selectively remove or receive the individual channels to minimize crosstalk from other channels. For a DWDM system to operate efficiently, the transmitter and receiver of a given channel should be capable of being tuned with great accuracy and stability to the same wavelength.
Conventional DWDM communication networks typically use semiconductor lasers, which emit light at fixed wavelengths. Although fixed-wavelength semiconductor lasers are satisfactory in many ways, it is anticipated that DWDM communication network will use tunable lasers in future. Tunable lasers have at least two advantages over fixed-wavelength lasers. First, tunable lasers would allow the inventory of lasers needed to equip and maintain a DWDM communication network to be significantly reduced. For example, a single tunable laser could, in principle, replace the 80 lasers of different wavelengths that would otherwise be required to equip an 80-channel DWDM communication network. The second advantage of tunable lasers is that they allow the DWDM communications network of which they are part to be reconfigured dynamically. In this way, the communications network can be controllably modified to accommodate unusual traffic patterns created by special events or by faults in parts of the network.
Another important application for a tunable laser is as a swept local oscillator in an optical spectrum analyzer.
Tunable lasers are being developed in response to the anticipated needs just described. In particular, tunable lasers that have no tuning mechanisms external to the semiconductor device are under development. However, such tunable lasers, in general, suffer from the disadvantage of having a limited tuning range.
Tunable external cavity lasers (ECLs) have also been proposed for use as tunable lasers in DWDM telecommunication systems and other applications. Tunable ECLs incorporating a semiconductor optical gain medium are described, for example, by Day et al. in Widely Tunable External Cavity Diode Lasers, 2378 SPIE, 35-41. The tunable ECLs disclosed by Day et al. incorporate a modified laser diode that has an anti-reflective coating on one facet thereof to cause the modified laser diode to operate as an optical gain medium and not as a laser. The uncoated facet defines one end of the external optical cavity. Light emitted from the coated facet is collimated by a collimating lens and the first beam portion is diffracted by a diffraction grating towards a mirror that defines the other end of the external optical cavity. The diffraction grating is rotated about an axis to tune the wavelength λ of the ECL. The ECL will lase at a wavelength selected by the grating provided that the selected wavelength within the modified laser diode's spectral gain region and the optical path length of the external cavity is an integral multiple of the selected wavelength. Tuning may also involve adjusting the length of the external cavity by moving the laser diode axially to change the length of the external optical cavity.
A tunable ECL employing a diffraction grating located in an external cavity is disclosed in U.S. Pat. No. 5,172,390 of Mooradian. This ECL requires a complex grating alignment system that significantly increases the cost of the device. Moreover, the tunable ECL disclosed by Mooradian and other similar tunable ECLs have a cavity length typically ranging from 25 millimeters (mm) to over 100 mm. This is in contrast to the much smaller (≦1 mm) optical cavity length of the FP lasers and DFB lasers described above. As a result, such tunable ECLs are typically much larger in size than fixed-wavelength FP lasers and DFB lasers.
FIG. 1 illustrates a tunable ECL known in the art as a Littman ECL. ECL 10 is composed of a modified laser diode 11, a converging lens 12, a diffraction grating 14 and a tuning mirror 16. The laser diode serves as an optical gain medium and is modified in that it has an anti-reflective coating on its front facet 19. The modified laser diode retains its reflective rear facet 18. The rear facet of the modified laser diode and the tuning mirror define opposite ends of optical cavity 13. The tuning mirror is mounted on an arm (not shown) that is controllably rotatable about a pivot 30 to tune the wavelength of the light generated by the ECL and to vary the optical path length of the optical cavity.
Diffraction grating 14 has a diffracting surface 15 and tuning mirror 16 has a reflecting surface 17. The diffraction grating and the tuning mirror are arranged so that tangents to the diffracting surface and the reflecting surface, respectively, intersect at pivot 30. Modified laser diode 11 is located such that a tangent to rear facet 18 passes through the pivot.
Modified laser diode 11 is capable of generating light over a broad range of wavelengths. Light emitted by the modified laser diode is collimated by converging lens 12 to form an incident beam portion 20. The incident beam portion is incident on diffracting surface 15 of diffraction grating 14 at an angle of incidence θI. In this disclosure, angles of incidence, angles of diffraction and angles of reflection are measured relative to the normal to the respective diffracting or reflective surface. The diffraction grating diffracts the incident beam portion at an angle of diffraction θD to provide a diffracted beam portion 22. The angle of diffraction depends in part on the wavelength of the light.
At a wavelength at which the angle of diffraction θD is equal to the angle between the reflective surface 17 of tuning mirror 16 and the diffracting surface 15 of diffraction grating 14, diffracted beam portion 22 is incident on reflecting surface 17 at an angle of incidence of zero. The reflecting surface reflects the light incident on it at an angle of incidence of zero back towards modified laser diode 11 as a return beam. The return beam travels along a path that is the reciprocal of the path of the emitted beam, i.e., along the paths of incident beam portion 20 and diffracted beam portion 22. Converging lens 12 focuses the return beam on modified laser diode 11.
Diffraction grating 14 and tuning mirror 16 collectively constitute a wavelength filter. At a given angle of rotation of the tuning mirror about pivot 30, only one wavelength of the light diffracted by the diffraction grating is incident on reflective surface 17 at an angle of incidence of zero. Only light of this wavelength will fully return to modified laser diode 11 after reflection, and only light of this wavelength is able to stimulate the modified laser diode to generate light. Accordingly, the modified laser diode only generates light of this wavelength. Moreover, ECL 10 is structured such that, at the given angle of rotation of the tuning mirror about the pivot, the optical path length of optical cavity 13 is an integral multiple of the selected wavelength, so that the return beam, after reflection by the reflective back facet 18 of the modified laser diode, will be in phase with the emitted beam emitted by the modified laser diode.
For a given pitch pg of diffraction grating 14 and a given angle of incidence θI of incident beam portion 20 on the diffraction grating, the wavelength λ at which the diffraction angle θD is such that the angle of incidence on reflecting surface 17 is zero is given by the following relationship:λ=pg[sin θD+sin θI],where the pitch pg is the distance between corresponding points on adjacent grooves in diffracting surface 15.
To provide continuous wavelength tuning without the number of wavelengths in optical cavity 13 changing, tangents to the diffracting surface 15 of diffraction grating 14, the reflective surface 17 of tuning mirror 16, and the reflective rear facet 18 of modified laser diode 11 should intersect at pivot 30, as shown in FIG. 1. To meet this condition, as ECL 10 is tuned, the arm (not shown) on which the tuning mirror is mounted is rotated about pivot 30 so that the tangent to the reflecting surface always passes through the pivot. A change in the number of wavelengths in the optical cavity that occurs as an ECL is tuned is known in the art as a mode hop.
In practice, a Littman ECL, such as ECL 10 shown in FIG. 1, only provides continuous wavelength tuning without mode hops when the index of refraction is homogeneous throughout the external cavity 13. In a practical embodiment, the external cavity contains elements with different indices of refraction. Specifically, the refractive index of the semiconductor portion of modified laser diode 11 is typically about 3.5 and the index of refraction of converging lens 12 is about 1.5. Air, a gas or a vacuum, each with a refractive index of approximately unity, constitutes the remainder of the optical path. To provide continuous tuning without mode hops, an additional mechanism (not shown) is provided to adjust the position of the modified laser diode and the converging lens along the path of incident beam portion 20 to compensate for the regions of different refractive index in the optical path. With adjustment of the modified laser diode and the converging lens, and the above-described arrangement of the diffracting surface 15 of diffraction grating 14, the reflecting surface 17 of tuning mirror 16 and pivot 30, continuous wavelength tuning without mode hops can be obtained.
For ECL 10 to lase, the return beam must return to the active area of modified laser diode 11. The active area is small, typically about 1.5 μm wide by 1 μm high at the front surface of the modified laser diode. The ability of the return beam to return to the active area of the modified laser diode is characterized by the angle of incidence of diffracted beam portion 22 on the reflective surface 17 of tuning minor 16. Light that is incident on the reflective surface with an angle of incidence of zero will return to the active region of the modified laser diode.
The angle of incidence of diffracted beam portion 22 on reflective surface 17 can be regarded as having two orthogonal components, a yaw component and a pitch component. The yaw component is the component of the angle of incidence in the plane in which diffraction grating 14 diffracts the light emitted by modified laser diode 11 and the pitch component is orthogonal to the yaw component. Component alignment and other errors that affect the yaw component of the angle of incidence merely change the wavelength of the light generated by ECL 10 at a given angle of rotation of tuning mirror 16 about pivot 30.
Component alignment and other errors that result in the angle of incidence having a non-zero pitch component have more serious consequences, however. When the pitch component of the angle of incidence exceeds a threshold value, the focused return beam will not return fully to the active area of modified laser diode 11. This increases the threshold current of the ECL. When the value of the pitch component of the angle of incidence exceeds a critical value, greater than the threshold value and typically about 0.005 mrad, the ECL will no longer lase.
Tunable ECLs of the type just described typically incorporate a folding mirror to eliminate mechanical interference between the housing in which modified laser diode 11 is mounted and the pivot bearing of the arm on which tuning mirror 16 is mounted. Incorporating a folding mirror may also allow the physical size of the tunable ECL to be reduced. However, errors in the alignment of the folding mirror can increase the variability of the pitch component of the angle of incidence. The need to accurately align the folding mirror further increases the difficulty of aligning and manufacturing the ECL.
Thus, what is needed is a tunable ECL having a simplified adjustment.